The present invention relates to series-resonant power converters and, more particularly, to a series-resonant converter topology and method of commutation for driving a switched reluctance motor (SRM).
The switched reluctance motor (SRM) has become an attractive alternative to conventional AC and DC motors in many industrial applications. This trend is a result of several distinct advantages in SRMs. For instance, the SRM is simple in design and lends itself to low cost construction. In operation, the SRM demonstrates an absence of shoot-through faults and has a high torque/inertia ratio.
Notwithstanding the above-described advantages of SRMs, the cost and performance of an SRM system is highly dependent on the selected power converter topology and commutation method used in powering the motor. The power converter for an SRM drive should be designed with the following objectives:
1. accurate yet high-speed switched power control for good drive performance, high efficiency and reliability; PA1 2. lower power (VA) rating for lower cost components; and PA1 3. minimum number of switches.
Soft-switching (a.k.a. resonant) power converters excel in each category, and they are fast becoming the favored alternative for static power conversion. The active devices in soft-switching converters are switched at zero-voltage or zero-current intervals. This way, they incur far lower switching losses than their hard-switched counterparts.
For example, U.S. Pat. No. 4,730,242 to Divan discloses a parallel resonant link converter in which the active switching devices are switched at times of zero-voltage intervals. Since the switching losses are comparatively small, commutation may occur at higher frequencies. However, the link current is bi-directional. Consequently, costly bi-polar self-commutated switching devices are required.
Series-resonant current source converters were developed in answer to the problem. With series-resonant converters, the switching of the active devices occurs at times of zero-current intervals rather than zero-voltage. For example, U.S. Pat. No. 4,942,511 to Lipo et al disclose a series resonant DC link converter. The natural turn-off ability of the switching devices is used for commutation. Hence, the converter can be implemented using less expensive uni-directional self-commutated switching devices such as thyristors.
It would be greatly advantageous if the series-resonant DC static power conversion circuits could be adapted for driving an SRM.